Bacterial adhesion and biofilm formation were studied in a continuous flow annular biofilm reactor using two commercial polyamide composite membranes - a relatively hydrophilic and smooth nanofiltration (NF) membrane and a relatively hydrophobic and rough reverse osmosis (RO) membrane. Multiple probe-liquid contact angle analyses confirmed that both membrane surfaces had nearly identical chemical properties after exposure to bacteria-free nutrient media - in particular the organic-fouled membranes were measurably more polar and hydrophilic. However, numerous bacterial microcolonies formed at discrete locations on both membranes, although more bacteria adhered to the RO membrane apparently due to its rougher surface. Cleaning agents such as sodium dodecyl sulfate and sodium hydroxide did not effectively remove biofilms adhered to either membrane surface. The adhesive bonds formed between the bacterial exopolymers and membrane surfaces were only broken and the membranes effectively cleaned when sodium hypochlorite was applied. Contact angle analyses revealed that exposure to cleaning chemicals altered membrane surface chemistry, but bacteria rapidly re-attached and formed new microcolonies on cleaned membranes. (C) 2010 Elsevier B.V. All rights reserved.